1. Field of the Invention
This invention relates to a composite thermostat metal (bimetal) and more particularly to a composite thermostat metal that combines qualities of high flexivity and corrosion resistance.
2. Description of the Prior Art
Composite thermostat metals are made of metallurgically bonded layers of metals having comparatively high and low coefficients of thermal expansion. When the temperature of thermostat metal is changed, the differences in the thermal expansion of the several layer materials results in a stress in the bimetal which is relieved by flexing. This flexing or mechanical movement induced by a temperature change can be made use of directly by configuring the bimetal to act as a switch or valve or indirectly by positioning the bimetal to act upon an auxiliary device. The intrinsic ability of an alloy combination to deflect in response to a temperature change is defined as the "flexivity" of the thermostat metal and the amount of flexivity of a given thermostat metal is substantially a direct function of the differences in the coefficients of thermal expansion of the several alloys in the composite thermostat metal. To achieve high flexivities, metal alloys are selected to have relatively large differences in their coefficient of thermal expansion. Other properties such as thermal response function and mechanical stiffness over desired operating temperature ranges, susceptability to metallurgical bonding processes, and resistance to corrosive environments are also of importance.
The terms "thermostat metal" and "flexivity" are more fully defined in ASTM B388 (75) paragraph 3.1 and ASTM B106 (78) respectively.
This invention is concerned with thermostat metals having relatively high levels of flexivity that can be used in corrosive environments. In many common applications for thermostat metals the corrosion resistance of conventional thermostat metals is quite sufficient. There are other applications in which the thermostat metals is exposed to hostile environments which requires care in the selection of the component alloys. An example of the latter is a damper adapted to be permanently installed in the flue of a combustion heating device such as a fireplace or furnace. In this application (see for example U.S. Pat. No. 3,510,059) the thermostat metal is configured to function as a damper which, when cool lays flat across the flue and restricts the flow of gases but, when heated by combustion gases, flexes (curls up) to open the flue. In this application the thermostat metal must be able to resist for many years the corrosive effects of volatile combustion products, including water vapor and acid forming gases, at temperature as high as 400.degree. C. Another demanding application utilizes a valve fashioned from a thermostat metal to control the flow of fluids through a conduit. One interesting application made of a device of this sort is to proportion the flow of hot and cold water so as to deliver water, as to a shower head, at a constant temperature.
The prior art recognizes that higher degrees of corrosion resistance can be obtained if the thermostat metal is made from components that are resistant to corrosive attack. For example, a number of stainless steel alloys are available which have high coefficients of thermal expansion. These are suitable for use as high expansive side (HES) of a thermostat metal and include AISI alloys such as types 302, 309, 316, and 385. Examples of other corrosion resistant alloys which are suitable HES materials include those comprised of 18% nickel, 11% chrome, balance iron, and 25% nickel 8% chrome balance iron. All of the above alloys have coefficients of thermal expansion roughly in a range of from about 9 to about 11.times.10.sup.-6 /.degree. F.
Unfortunately, the selection of a corrosion resistant alloy having a low coefficient of thermal expansion suitable for use on the low expansive side (LES) is much more limited. For example AISI alloy type 430, an alloy which has sometimes been used as an LES has a coefficient of thermal expansion of 5.8.times.10.sup.-6 /.degree.F. This compares rather unfavorably to Invar, by far the most commonly used and best (except for its poor resistance to corrosion) alloy for the LES which has a coefficient of thermal expansion of only 1.1.times.10.sup.-6 /.degree. F. The fivefold difference in the coefficient of thermal expansion between AISI 430 and Invar greatly reduces the flexivity and utility of a thermostat metal when AISI 430 is used as the LES.
Corrosion resitant thermostat metals are also made by capping both sides of a high flexivity metal with corrosion resistant alloys. The difficulty encountered here, however, is that, if the cap layers are to be effective, they must be of significant thickness which necessarily will reduce the overall flexivity of the composite. It should be understood that thermostat metals are commonly reduced to the desired product thickness by cold rolling and 20% to 50% cold work is typical to obtain a useful product thickness in a range of 0.005 inches to 0.050 inches. Unless the cap metal is of a similar hardness as the alloy to which it is bonded, there is a tendency for fissures to develop in the cap layer during cold rolling which will expose portions of the surface of the surface of the underlying alloy. This development of fissures is particularly pronounced when AISI 430 is used to cap Invar since AISI 430 is much harder than Invar in all states of temper. The problem can be overcome by using a thick cap layer, but then a substantial reduction in the flexivity of the thermostat metal must be accepted.
Another method of providing increased corrosion resistance is to make a thermostat metal comprised of a high flexivity stainless steel HES, a conventional Invar LES and to cap only the LES with a corrosion resistant alloy. This is subject to much of the same objections as mentioned immediately above in that the difference in hardness of a typical stainless alloy such as AISI 430 makes it difficult to cold roll it in thin layers over Invar without developing fissures. The thickness required to avoid fissures, greatly reduces the flexivity of the thermostat metal.